A characterizing feature of such multi-product production plants for polyester polyols is that a wide range of polyester polyols are produced in a limited number of reaction vessels, wherein the number of products produced normally exceeds the number of vessels available several times. Because undesired cross-contamination has to be avoided, the sequence, in which different polyester polyols may be produced in the same vessel is subject to certain constraints which may severely restrict allocation of resources. After production, the polyester polyols are transferred into tank trucks, storage vessels or drums via a fixed pipe system. Undesired cross-contamination of the various products must again be avoided at this point.
According to prior practices in such cases, a full cleaning operation has to be performed, by rinsing the vessel(s) and optionally the corresponding pipes with hot aqueous sodium or potassium hydroxide solution. This step is conventionally followed by a neutralization step using dilute acetic acid and a final rinsing process using water. These steps disadvantageously result in a worthless rinsing liquid, which has to be disposed off at considerable cost and has an impact on waste water treatment plants.